This invention relates generally to rotatable alternating current (a-c) dynamoelectric machines of the synchronous type, and it relates more particularly to a system for starting such a machine for the purpose of "cranking" a large diesel engine whose crankshaft is coupled to the rotor of the machine.
Large self-propelled traction vehicles such as locomotives are commonly equipped with "electrical transmissions." Such a transmission typically comprises a main synchronous machine (sometimes referred to as a traction alternator) for supplying electric energy to a plurality of direct current (d-c) traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the vehicle. The rotor of the alternator is mechanically driven by the crankshaft of a thermal prime mover (typically a 12 or 16-cylinder diesel engine) on board the vehicle. When excitation current is supplied to a field winding on the rotor of the alternator, alternating voltages are generated in its 3-phase stator windings. The alternating voltage output of the stator windings is rectified and applied to the armature windings of the traction motors. The output power of the alternator is regulated or varied by suitably controlling the strength of its field excitation and the rotational speed of the engine.
The above-summarized self-propelled vehicle in practice will sometimes be out of service with its engine not running. In order to return the vehicle to service, appropriate means needs to be provided for turning the crankshaft until the engine starts running. Heretofore this engine "cranking" function was usually performed by an auxiliary d-c generator that has a rotor mechanically coupled to the crankshaft and that is temporarily operated in a "motoring" mode in which its armature winding is energized by the vehicle battery. A 1969 British Pat. No. 1,164,957 discloses that the main alternator itself can be used as a synchronous motor for engine cranking, thereby saving the space, weight, cost, and mechanical wear of a separate starting motor. In the latter case, however, the starting system needs to include suitable means for converting the unidirectional battery current to the variable frequency, 3-phase alternating current that is required by the traction alternator when operating in a motoring mode.
In an engine cranking system utilizing the prior art combination of a vehicle battery, an electric power converter, and the above-mentioned 3-phase a-c synchronous machine, the initial output torque of the rotor of the machine (and hence the magnitude of current in the stator windings) needs to be relatively high in order to start turning the crankshaft of the engine. As the rotor accelerates from rest, less torque (and current) will be required, while the fundamental frequency of load current will increase with speed (revolutions per minute). In this cranking mode of operation, the converter supplies the machine with current of properly varying magnitude and frequency until the engine crankshaft is rotating at a rate that eouals or exceeds the minimum speed at which normal running conditions of the engine can be sustained.
One particular converter that is well suited for use in the foregoing combination is known as a current-fed "third harmonic" auxiliary impulse commutated inverter. The principles of commutation and a typical practical application of such an inverter were described in a technical paper entitled: "Analysis of a Novel Forced-Commutation Starting Scheme for a Load-Commutated Synchronous Motor Drive," which paper was presented by R. L. Steigerwald and T. A. Lipo at the IEEE/IAS annual meeting held in Los Angeles, Calif. on Oct. 2-4, 1977. The Steigerwald and Lipo paper was reprinted in IEEE TRANS. Vol. IA-15, No. 1, Jan/Feb 1979, pgs. 14-24, and it is expressly incorporated herein by reference.
In essence, a third harmonic auxiliary impulse commutated inverter comprises six main unidirectional conduction controllable electric valves, such as thyristors, that are interconnected in pairs of series aiding, alternately conducting valves to form a conventional 3-phase, double-way, 6-pulse bridge between a pair of d-c terminals and a set of three a-c terminals. The d-c terminals of the bridge are adapted to be connected to a suitable source of relatively smooth direct current, such as a large, multicell, heavy duty electric storage battery. The a-c terminals of the aforesaid bridge are respectively connected to the different phases of a 3-phase electric load circuit which typically comprises star-connected 3-phase stator windings of a large synchronous motor.
To supply the load circuit with 3-phase alternating current, the six main valves of the inverter are cyclically turned on (i.e., rendered conductive) in a predetermined sequence in response to a family of "firing" signals (gate pulses) that are periodically generated in a prescribed pattern and at desired moments of time by associated control means. To periodically turn off the main valves by forced commutation, the inverter is provided with an auxiliary circuit comprising a precharged commutation capacitor and at least seventh and eighth alternately conducting unidirectional controllable electric valves that are arranged to connect the capacitor between the neutral or common point of the 3-phase a-c load circuit and either one of the d-c terminals of the bridge.
During each full cycle of steady state operation of a third harmonic inverter, each of the valves in the auxiliary commutation circuit is briefly turned on three separate times. More particularly, the 7th valve is fired at intervals of approximately 120 electrical degrees, and the 8th valve is fired at similar intervals that are staggered with respect to the intervals of the 7th valve, whereby one or the other auxiliary valve is fired every 60 electrical degrees. When an auxiliary valve is turned on, the commutation capacitor is effectively placed in parallel with one phase of the load circuit and a first one of the two main valves which are then conducting load current. Initially, the capacitor voltage magnitude is higher than the amplitude of the line-to-neutral voltage that is developed across the inductive load, and its polarity is such that the capacitor starts discharging. Consequently current is forced to transfer (commutate) from the first main valve (i.e., the offgoing or relieved valve) to a parallel path including the turned-on auxiliary valve and capacitor. The rate of change of current during commutation will be limited by the load inductance.
After current in the offgoing main valve decreases to zero, the magnitude of capacitor voltage is still sufficient to keep that valve reverse biased for longer than its "turn-off time." As soon as the commutation capacitor is fully discharged, load current begins recharging it with opposite polarity. Once the commutation capacitor is recharged to a voltage magnitude exceeding that of the line-to-neutral load voltage, the next (oncoming) main valve in the bridge is forward biased ano can be turned on, whereupon load current commutates from the turned-on auxiliary valve and commutation capacitor to the oncoming main valve. This causes the auxiliary valve to turn off and completes the commutation process. The capacitor is left with voltage of proper polarity and sufficient peak magnitude for successful commutation of the second one of the first-mentioned two conducting main valves when the opposite auxiliary valve is turned on approximately 60 degrees later. It will be apparent that there are six intervals of commutation per cycle, the direction of current in the commutation capacitor during each interval is reversed compared to the preceding interval, and therefore the fundamental frequency of the alternating capacitor current equals the third harmonic component of load frequency.
As is pointed out in the referenced Steigerwald and Lipo paper, one practical application of a current-fed third harmonic auxiliary commutated inverter is in an adjustable speed a-c drive system where the 3-phase star-connected stator windings of a synchronous machine are supplied with variable frequency a-c power by the inverter which needs to be forced commutated in order to start the machine. As the angular velocity. of the machine rotor increases from 0, the alternating voltages developed across the stator windings (i.e., the back electromotive force) increase in amplitude. Consequently the magnitude of current (and hence accelerating torque) tends to decrease, as does the magnitude of the forward bias voltage on the main valves of the inverter at the times their respective firing signals are generated. In order to assure proper turn on of the valves and continuous acceleration of the rotor, it is necessary to control the angle between the field magnetomotive force (mmf) and the stator mmf. This can be done by known techniques for regulating "torque angle," but such techniques are relatively complex to implement.